This invention relates to a combinatorial weighing or counting method in a combinatorial weighing or counting apparatus having a plurality of weighing machines. More particularly, the invention relates to a combinatorial weighing or counting method in an apparatus of the type which discharges weighed articles from selected weighing machines corresponding to respective bits of a combination bit pattern obtained as a result of a combination computing operation, which method assures that certain weighing machines will not remain unselected over an extended period of time, thereby eliminating extended residence of the articles within those weighing machines.
In the prior art, a combinatorial weighing or counting apparatus is available for accurately weighing out and dispensing weighed articles of a preset target weight or preset target number. Such apparatus is equipped with a plurality of weighing machines, typically ten in number, for independently weighing articles supplied to them. Thus, as a result of the independent weighing operations, a plurality of weight values are obtained. The apparatus operates by forming all possible combinations of these weight values, adding the weight values in each combination to provide a multiplicity of sums, selecting the sum which is equal to a target value or closest to the target value within preset allowable limits, and discharging the articles from the weighing machines belonging to the combination that gave the selected sum.
In actually using an apparatus of the above-described type, there are instances where certain weighing machines are not selected, and hence are not permitted to discharge their articles, even over an extended period of time. Theoretically, if we assume that half of the weighing machines are selected each time, then the probability of a weighing machine being selected each time should be 50% (one out of two). Accordingly, the probability of a weighing machine not being chosen in, say, five consecutive selection cycles is 3.125% (one out of 32), so that we would expect a given weighing machine to be chosen within the five consecutive cycles. In actuality, however, we find that, depending upon the weight of the articles delivered to the weighing machines, a given weighing machine or machines may not be selected over a great many cycles, which can reach a figure of as high as 20 or more. When a weighing machine is not selected for a prolonged period of time, certain problems arise as will now be described.
One problem is that the weight sensor constituting a weighing machine experiences a variation in zero point with time owing to such factors as temperature change and drift. Ordinarily, the weight sensor is subjected to a zero adjustment with each discharge of articles or every predetermined number of discharge cycles, whereby the variation in zero point is corrected for. When a weighing machine is not selected, however, no correction is effected. If the weighing machine goes unselected for a prolonged period of time, therefore, the weighing error resulting from the change in zero point will grow large in magnitude.
Another problem is that the longer articles reside in the weighing hopper constituting a weighing machine, the more likely the articles are to attach themselves to the walls of the hopper. When such a condition occurs, not all of the articles are discharged from the weighing hopper so that the weight of the articles discharged obviously will be less than the measured weight. When the articles contain oil, grease or a similar fatty substance, moreover, the tendency for the articles to cling to the walls of the weighing hoppers becomes quite pronounced, especially when the fatty substance comes to the surface of the articles.
A third problem involves the weighing of frozen foods. Prolonged residence in a weighing machine will allow the surface ice on such articles to thaw, whereby the articles are likely to both spoil and become affixed to the walls of the weighing hopper.
A fourth problem arises in a case where the weighing machines are supplied with the articles by means of radiating troughs corresponding to respective ones of the weighing machines. Specifically, when the supply of articles from a trough to a corresponding weighing machine is suspended continuously, the articles in the trough gradually increase in quantity due to the action of the other troughs, until some of the articles begin falling from the trough onto a pool hopper, thereby temporarily upsetting the balanced feed of the articles.
A fifth problem involves the question of accuracy. The fact that a certain weighing machine is not selected over an extended period of time indicates that the weighing machine has not been supplied with the proper amount of articles and therefore is not actually participating in the combinatorial weighing operation. This diminishes the effectiveness of the combinatorial weighing operation and, hence, invites a decline in weighing accuracy.